Piezoelectric transformers allow the conversion of a supplied AC voltage as input voltage on an input side into a higher or lower AC voltage as output voltage on an output side of a piezoelectric element. The piezoelectric element is frequently constructed from a lead zirconate titanate compound (PZT). On the basis of an appropriate polarization of the input and output sides of the piezoelectric element, the latter is deformed, e.g., in the thickness direction when a sinusoidal AC voltage is applied on the input side on account of the inverse piezoelectric effect, as a result of which an oscillation is produced in the longitudinal direction of the piezoelectric element. This in turn produces a corresponding output voltage on the output side on account of the direct piezoelectric effect.
If the frequency of the applied input voltage matches the resonant frequency of the piezoelectric element, this results in an electromechanical resonance of the element, which means that the mechanical vibration reaches a maximum. In this manner, a very high output voltage can be produced on the output side of the piezoelectric element. One application involves, by way of example, operating a piezoelectric transformer as a plasma generator, wherein ionization of an operating gas flowing around the plasma generator takes place on account of a high output voltage on the output side of the plasma generator, so that a plasma is produced.
When operating a piezoelectric transformer, it is further always desirable to operate the transformer at a maximum efficiency. The maximum efficiency can only ever be achieved at one particular frequency. This frequency is dependent on numerous parameters, inter alia on the input voltage and the operating environment used. Finding the maximum efficiency therefore requires information from the component.
There are already multiple possibilities for the frequency regulation of conventional piezoelectric transformers. For example, the voltage on the secondary side (output voltage) can be considered. Another possibility is the use of an additional electrode on the transformer to obtain a feedback signal.
A disadvantage in the case of the first possibility is that every instance of the output voltage being tapped off influences the amplitude of the output voltage and hence the oscillatory response of the piezo element and finally the manner of operation of the piezoelectric transformer. Particularly when the piezoelectric transformer is used as a plasma generator, such a possibility for the frequency regulation would have a greatly adverse influence on the manner of operation of the plasma generator.
By contrast, the second variant has the disadvantage that it necessitates further contact connection of the piezoelectric component, which complicates the design.